Epson Perfection 3590 Photo
On page http://sane-project.org/sane-mfgs.html#Z-EPSON,
Perfection 3590 PHOTO support is "Basic" by snapscan but without the film scanner and does require firmware.
The sourceforge snapscan sane page - http://snapscan.sourceforge.net/#work - lists the 3490 and the 3590 but not the Photo versions. All have id of 0x04b8, 0x0122. Id string is "EPSON Scanner". Firmware is Esfw52.bin.
On page http://sane-project.org/man/sane-snapscan.5.html, under "Files", for Mint 19.3,
a) the config files are in /etc/sane.d, not /usr/local/etc/sane.d/
b) the backend library is at /usr/lib/x86_64-linux-gnu/sane/, not /usr/local/lib/sane/, (and must contain must contain - libsane-snapscan.so.....)Also, c) etc/sane.d/dll.conf specifies the backends in use and should have "snapscan" uncommented. d) firmware is located at /lib/firmware and must contain Esfw52.bin.
The config file /etc/sane.d/snapscan.conf should contain: # USB (Epson 3590) usb 0x4b8 0x122 and a link to the firmware: firmware Esfw52.bin
and any scsi entries can be commented-out as this is a usb scanner.
Hope this helps someone.